Many industrial processes generate substantial quantities of waste water, much of which is contaminated with toxic metal ions and therefore cannot be discharged into the environment without endangering the public health. There are many waste water treatment processes in use today, including ion exchange processes, hydroxide precipitation processes, and sulfide precipitation processes, to name just three. All of these processes have advantages that may recommend them for particular applications, and disadvantages that preclude their use for other applications. Ion exchange methods are useful where decontamination and recovery of precious metals is desirable. However, ion exchange resins are expensive, and ion exchange methods cannot ordinarily be used effectively for solutions containing high concentrations of ancillary salts. Hydroxide precipitation is inexpensive and effective to precipitate metals having insoluble hydroxides which are not redissolved in excess alkali. However, alkali hydroxides are poisonous and corrosive, and the excess hydroxide must be neutralized with acid before the treated waste water can be returned to the environment. Sulfide precipitation can be used effectively to precipitate contaminating metals which form insoluble sulfides. However, soluble sulfides form toxic liquids which can release one of the most toxic and unpleasant gases on exposure to acids, i.e., hydrogen sulfide, which can be released from some inorganic sulfides on exposure to moist air containing carbon dioxide.
U.S. Pat. No. 4,329,224 (Kim) discloses a precipitation method for decontaminating water based upon treatment of the water with an aqueous calcium sulfide composition. Water to be decontaminated is treated with dissolved calcium sulfide, i.e., with a solution of dissolved calcium hydroxide through which hydrogen sulfide gas has been passed. Treating dissolved calcium hydroxide, Ca(OH).sub.2, with hydrogen sulfide gas yields dissolved calcium bisulfide Ca(HS).sub.2. The bisulfide ions act as the source of sulfide ions which precipitate the sulfides of undesirable metals.
It will be recognized by those skilled in the art that the Kim process is inherently dangerous and unpleasant. Hydrogen sulfide is an evil-smelling gas with the odor of rotten eggs, and is detectable at very low concentrations. High concentrations of hydrogen sulfide are extremely hazardous, and a few hundred parts per million can produce death in a short time. At high concentrations the gas is also combustible. Dissolved sulfide solutions are dangerous since they are inherently toxic and since they release hydrogen sulfide gas upon exposure to acids. Carbon dioxide and moisture can react in the air to form carbonic acid, which acid can release hydrogen sulfide from many aqueous soluble sulfide solutions exposed to air. Thus, any process using compressed hydrogen sulfide, or soluble sulfides, is inherently dangerous.
Kim describes his calcium sulfide water treatment as a two-stage process in which "nontoxic" metals are removed by hydroxide precipitation in the first stage and "toxic" metals are removed by sulfide precipitation in the second stage. However, the fact is that some toxic metals, such as thorium, can be removed by hydroxide precipitation in the first stage, while other toxic metals, such as barium, are not removed by sulfide precipitation in the second stage. The Kim process is based upon an inaccurate principle since nature has not classified nontoxic or toxic metals as those that can or cannot be precipitated by alkali, respectively.
It is clear to those skilled in the art that a more economical and safer waste water treatment method is required, which does not require the use of expensive, unstable, combustible, or toxic materials. Calcium sulfide is stable only in the dry form and is in relatively short supply. These problems of supply, stability, and toxicity are critical considering the immense amounts of this chemical required for large-scale water treatment processes.